Hot strips or heavy plates from a denitrated steel and method for their manufacture

Abstract

An improved method for the manufacture of hot strips or heavy plates from a denitrated steel composed of 0.04 to 0.16% carbon, 1.25 to 1.90% manganese, 0.2 to 0.55% silicon, 0.004 to 0.020% phosphorus, 0.002 to 0.015% sulfur, 0.02 to 0.08% aluminum, 0.02 to 0.08% niobium, the remainder iron and possibly contaminants, including the steps of having the hot strips or plates leaving the last finishing stand of rolls at a temperature of 750° C. to 820° C., cooling the hot strips or plates to an intermediates temperature of 450° C. to 500° C. at a cooling rate of 2° to 10° C./s and then slowly cooling the hot strips or plates in air to room temperature in a coil or in a pile.

We claim: 
     
       1. In a method for the manufacture of hot strips or heavy plates from a denitrated steel composed of 0.04 to 0.16% carbon, 1.25 to 1.90% manganese, 0.2 to 0.55% silicon, 0.004 to 0.020% phosphorus, 0.002 to 0.015% sulfur, 0.02 to 0.08% aluminum, 0.02 to 0.08% niobium, the remainder iron and possibly contaminants, the improvement comprising the steps of having the hot strips or plates leaving the last finishing stand of rolls at a temperature of 750° C. to 820° C., cooling the hot strips or plates to an intermediate temperature of 450° C. to 500° C. at a cooling rate of 2° to 10° C./s and then slowly cooling the hot strips or plates in air to room temperature in a coil or in a pile. 
     
     
       2. Method according to claim 1, wherein the steel is alloyed with 0.02 to 0.10% addition of vanadium. 
     
     
       3. In a method for the manufacture of hot strips or heavy plates from a denitrated steel composed of 0.04 to 0.16% carbon, 1.25 to 1.90% manganese, 0.02 to 0.55% silicon, 0.004 to 0.020% phosphorus, 0.002 to 0.015% sulfur, 0.02 to 0.08% aluminum, 0.02 to 0.08% niobium, as well as addition of 0.15 to 0.35% molybdenum, 0.10 to 0.30% chromium and/or 0.30 to 0.90% nickel, alone or in combination, the remainder iron and possibly contaminants, the improvement comprising the steps of having the hot strips or plates leaving the last finishing stand of rolls at a temperature of 750° C. to 850° C., cooling the hot strips or plates to an intermediate temperature of 450° C. to 500° C. at a cooling rate of 2° to 10° C./s and then slowly cooling the hot strips or plates in air to room temperature in a coil or in a pile. 
     
     
       4. Method according to claim 3, wherein the steel is alloyed with 0.02 to 0.10% addition of vanadium. 
     
     
       5. Method according to claim 3 or 4, wherein the finishing temperature lies between 750° C. and 820° C. 
     
     
       6. Method according to claim 3, wherein the steel includes an addition of 0.002 to 0.08% zirconium. 
     
     
       7. Method according to claim 3, wherein the steel includes an addition of 0.004 to 0.051% cerium. 
     
     
       8. Hot strips or heavy plates manufactured according to the method of claim 1 or 3. 
     
     
       9. Hot strips or heavy plates according to claim 8, wherein the steel exhibits a ferritic-perlitic structure. 
     
     
       10. Hot strips or heavy plates according to claim 8, wherein the ratio of Cvmax to Cv100 lies between 1.0 and 1.3. 
     
     
       11. Hot strips or heavy plates according to claim 8, wherein the ratio of Cvmax to Cv100 lies between 1.3 and 2.0. 
     
     
       12. Hot strips or heavy plates according to claim 8, having a maximum impact tenacity value of 280 J/cm 2  at a test temperature of -20° C. 
     
     
       13. Hot strips or heavy plates according to claim 8, having a maximum impact tenacity value of 230 J/cm 2  at a test temperature of -40° C.